The unprecedented rise in the number of twins born in the U.S. over the past 30 years is a major public health concern because twin pregnancies have high rates of serious maternal and neonatal complications. Gestational weight gain (GWG) is a potentially modifiable risk factor for adverse outcomes common to twin gestations. Unfortunately, data on optimal GWG for twin pregnancies is extremely limited. The 2009 Institute of Medicine Committee published provisional GWG guidelines based only on the interquartile range of GWG observed in a single study of 2234 self-selected mothers who delivered twins weighing at least 2500 g at e36 weeks gestation. The proposed study will inform evidence-based GWG guidelines for twin gestations by evaluating the total amount and pattern of weight gain in twin pregnancies that is associated with optimal health outcomes for mothers and offspring. This project directly responds to PA-12-061 for research on the effects of maternal nutrition on mothers, infants and children. Our study design will employ existing electronic records from 2880 twin pregnancies at Magee-Women's Hospital in Pittsburgh, Pennsylvania; vital records from a population-based cohort of 12,300 twin pregnancies in Pennsylvania; and data from a nationally-representative longitudinal birth cohort of 1658 twins. Our project seeks to determine the association between total GWG and adverse outcomes for mothers (gestational diabetes, preeclampsia, unplanned cesarean delivery, weight retention at 5 years postpartum) and offspring (stillbirth, infant mortality, spontaneous preterm birth <32 weeks, abnormal fetal growth, child overweight, and poor cognitive development). An innovative method to assess total GWG will be developed that removes the bias in the existing measures' correlation with gestational duration. This project will also examine the relationship between maternal GWG trajectory and adverse perinatal outcomes. A novel growth curve analysis method will be used to determine the contribution of timing, amount, and velocity of GWG to poor pregnancy and birth outcomes. Successful completion of these aims will provide the most comprehensive evaluation of GWG and adverse outcomes for women carrying twin pregnancies across all BMI categories to-date. This study will be the first to assess the consequences of high GWG in twin pregnancies on longer-term risks of obesity in postpartum mothers and their children, and will fill major gaps in knowledge on the optimal GWG for severely obese women. This research will provide a strong foundation for future evidence-based guidelines, which will guide medical practice and public health policy for these high risk pregnancies.